Thousands of bicycles are stolen each day. Existing and presently used locking devices for bicycles invariably employ a wire cord or chain which, together with a key or combination padlock, is used to secure the bicycle to a post, bicycle rack or the like. In view of the number of bicycles stolen each day, this method of locking is not effective. The external locking means is too easily and readily compromised using a commonly available wire, chain or bolt cutter handtool. A principal object of the present invention is to provide an effective locking system for bicycles which is internally secured within the frame of the bicycle and not subject to compromise by commonly available handtools. Another object of the present invention is to provide a locking system for bicycles which has both an internally secure locking mechanism and an external locking mechanism to prevent removal of a bicycle.